Generating clean stable dc power for sensitive equipment from an ac source, single phase or three phase, is a common problem. The most simple solutions utilize bridge rectifier circuits and large filtering capacitors. Simple bridge rectifiers, however, have input current waveforms that are not sinusoidal and are highly distorted, resulting in very poor input power factors and total harmonic distortion. The use of a boost converter having a single switch, or multiple parallel switches, between the bridge rectifier and the dc output can significantly improve the input power factor and reduce the total harmonic distortion. The objectives of the boost converter are to shape the input line current to become a square wave which can be done using constant peak current-mode control with a switching duty cycle of less than 50%, and to regulate the output voltage to ensure that it is constant.
It often becomes necessary or desirable, for example with increased power levels, to augment the single boost converter, which has multiple switches connected in parallel, with additional boost converters which are interleaved with the first. The interleaving of two boost converters results in an overall converter size reduction due to the reduced size of the energy-storage boost inductors, differential-mode electro-magnetic interference ("EMI") filters, and the output filter capacitors. Interleaving also effectively increases the switching frequency without increasing the switching losses of power switching devices.
The control scheme of conventional interleaved boost converters for a three-phase, six-diode input bridge, consists of two PWM controllers, each controlling one voltage regulation loop, and one current control loop, which can be either peak current or average current controlled. One of the PWM controllers acts as a master controller and the other acts as a slave controller where the clock of the salve controller is synchronized to the master with the help of additional synchronization circuits.
The problem with this control scheme is that the parameter variation of the synchronization circuit components and other circuit elements will cause the duty cycle of the slave boost converter to be different from that of the master boost converter. Therefore, the two boost converters may operate at different duty cycles and the two boost switchings may not be the expected 180 degrees out of phase. The result is a degradation in the anticipated effect on the input line current ripple cancellation and the output ripple cancellation. Further, the two boost converters will exhibit asymmetrical sharing of the current in addition to the use of more circuit components.
An interleaved boost converter system configured to use a single PWM controller would ensure the proper 180 degrees phase shift between the switching of the two boost switches and would result in symmetrical current sharing, thereby solving the problems in input line current harmonics, output ripple, and the asymmetrical sharing of current in the two boost switches seen with the master/slave controllers of the conventional scheme.